The purpose of this study is to study the function of the gastrointestinal tract in health and disease using a variety of techniques. The metabolism of the colonic bacteria is studied in situ within the colon by measurement of volatile bacterial metabolites which are absorbed into the blood supply of the colon and then excreted on the breath. The blood flow to the villus lining of the intestinal tract is studied by means of microspheres which lodge in the villus capillaries. The ability of the large intestine to serve as an absorption site for materials not absorbed in the small bowel is investigated by perfusion techniques and study of breath gas excretion.